Field of the Invention
The invention relates to a process and an installation for heating a metallic strip to a set temperature needing the presence of a reducing combustion atmosphere (lack of fuel in comparison with stoechiometric quantity).
The invention is described as for its application to heating of a metallic strip in the form of a strap transmitted to an annealing installation, notably for making galvanized steel.
Description of the Related Art
Various types of annealing installations for straps are already known.
FIG. 1 is a schematic view of a first example of a known installation for preheating a strap to a set temperature appropriate to an annealing installation. It comprises an oven delimiting a chamber 102 for preheating the strap 100 and a chamber 104 for heating the strap in order that, at the installation exit, the strap is at a set temperature, for example of 750° C.
In the preheating chamber 102, comprised between the oven entry 108 and a place 110 where post-combustion air from a duct 112 is injected, hot gases with no reducing power, so that the atmosphere in the oven, which could flow out through the entry 108, is not toxic (i.e. it contains no CO), flow from the heating part 104 to the proximity of the entry 108, in a direction opposite that of the strap 100. The heat exchanges between a surface and a current parallel to this surface are not very efficient, because of the presence of laminar layers. Consequently, the chamber 102 must have a great length.
In the combustion chamber 104, practically comprised between the air injection place 110 and the oven exit 114, burners 116 receive a fuel transmitted by a feed circuit 118 connected to each burner 116, and combustion air by a circuit 120 also connected to each burner. The air in the circuit 120 has been preferably preheated in a heat exchanger 122 in which also circulate the hot gases evacuated by a duct 124 from the proximity of the oven entry end 108.
In the heating chamber 104, the burners 116 are open fire burners working in a reducing combustion atmosphere, i.e. in the presence of CO. In the heating chamber 104, the strap temperature increases from a temperature around 350 to 400° C., near the air injection place 110, to a set temperature for example of 750° C. at the exit 114.
The just-described installation has some drawbacks.
First, because of the simple circulation of the hot gases in the direction opposite that of the strap in the preheating chamber 102, the heat exchange between the gases and the strip is not efficient, so that the chamber 102 has a very great length and the oven takes up thus much room.
Then, the whole system using open fire burners for forming the reducing combustion atmosphere with the air preheated by the exchanger 122 has a relatively low efficiency, in the order of 50%, so that the fuel consumption is high.
At last, the heat losses are important, essentially due to the evacuation of the gases, still at a high temperature, into the atmosphere, because the heat exchanger 122 which collects the heat of the gases evacuated by the duct 124 has a moderate efficiency.
In the document JP2002-294 347, it is also described another example of a known installation for preheating a strap, before annealing it, which does not have the drawback of taking up very much room at the floor level.
More precisely, this installation comprises a preheating chamber 202 with a small length, because it comprises a device 210 for preheating the strap by projecting hot gases in a direction perpendicular to the strap. This device comprises chambers whose wall, facing the strap, comprises a plurality of orifices projecting as many jets of hot gases. Such a device 210, sometimes called “plenum”, enables to get an efficient heat exchange over a small length.
This preheating chamber 202 is connected to a preheating chamber 204 in which the strap follows a zigzag path between radiant tubes. The preheating chamber and the heating chamber have the same protective atmosphere. The combustion gases evacuated by the radiant tubes, which are separated from the protective atmosphere in the chamber 204, are extracted by ducts 205 to a heat exchanger 26, with the help of a blower 207. In the heat exchanger 206, a protective atmosphere circulates under the control of a blower 208. The circulation circuit comprises two parallel connected ducts, one duct 209 feeding the device 210 for projecting hot gases onto the strap and another duct 211 comprising a control valve 212 which opens more or less for modulating the quantity of gas transmitted to the device 210 for projecting hot gases.
Thus, the set temperature sensor 218, which measures the temperature of the strap at the installation exit, enables to control the valve 212 in order that it regulates the quantity of gas able to circulate through the duct 209, and thus the preheating power of the projection device 210, in order that the set temperature, measured by the sensor 218, does not vary.
The installation, represented in FIG. 2, does not have the drawback of taking up much room, but it has other drawbacks.
First, the efficiency of the radiant burners used is not high.
Then, as the heat of the combustion gases is only partially collected by the heat exchanger 206, the installation has an energetic efficiency which is relatively reduced and does not exceed a value in the order of 50%.
Moreover, the whole installation, comprising both chambers and the gas circuit for feeding the chambers for projecting gases onto the strap, contains a protective atmosphere which can be toxic and then does not fulfill the “hygienic” requirements for the combustion, or which is inert and expensive.
At last, as the strap follows a sinuous path inside the preheating chamber, the installation is onerous and prone to failures.
Preheating units with “regenerative” burners are otherwise known, notably from the document JP-2001/304 539. The term “regenerative” indicates that, in a first phase, some heat extracted from combustion gases is accumulated, and, in a second phase, the accumulated heat is returned to the combustion air. Such units usually comprise two burners 301a, 301b which are mounted in tandem, the one working in a combustion mode in which some combustion air in a duct 316 circulates through a regenerative mass 306 before participating to a combustion, and the other working in a heat recuperation mode in which the combustion gases from the first burner circulate through its regenerative mass 306 and heat it. After a certain length of time, for example a fraction of a minute, both burners exchange their ways of working.